Recently, an optical network using a WDM (wavelength division multiplexing) transmission technique has been implemented to realize a large capacity communication. Also to realize a long distance transmission, a low noise optical amplifier is requested. Accordingly, a Raman amplification has been put into practical use as one of low noise optical amplifying methods in a long distance WDM transmission system.
The Raman amplification is realized by providing pump light for an optical transmission line which operates as an optical amplification medium. A Raman gain occurs on the wavelength shifted by a specified Raman shift with respect to the pump light. When a WDM optical signal is amplified, it is preferable that a gain is flat with respect to a wavelength in the signal wavelength band of the WDM optical signal. Therefore, the pump wavelength of the Raman amplification is determined so that, for example, the gain may be flat with respect to the wavelength in the signal wavelength band.
When the signal wavelength band is wide, a Raman amplifier having a wide wavelength band in which a gain is flat with respect to a wavelength is requested. To satisfy the request, a Raman amplifier having a plurality of different pump wavelengths has been proposed. In this Raman amplifier, for example, an amplified WDM optical signal is divided into a plurality of wavelength bands, and the optical power of each wavelength band is monitored. Then, the power of each of the plurality of pump wavelengths is controlled so that the optical power of each of the wavelength bands may be equal or substantially equal.
Note that the related technologies are described in, for example, Japanese Laid-open Patent Publication No. 2002-72262, Japanese Laid-open Patent Publication No. 2000-98433, Japanese Laid-open Patent Publication No. 2004-193640, and Japanese Laid-open Patent Publication No. 2008-182679.
In the Raman amplifier having a plurality of pump wavelengths, a plurality of monitor circuits are required to obtain equalized or substantially equalized optical power of each channel of a WDM optical signal as described above. Therefore, the production cost of the Raman amplifier becomes high.